The present invention relates to a tandem-type transfer device that is provided with a plurality of photoreceptor drums for respective hues and transcribes toner images formed on the respective photoreceptor drums to an intermediate transfer belt.
In conventional tandem systems, transfer of the toner images on the respective photoreceptor drums to the intermediate transfer belt has been carried out by applying a transfer bias to each of the photoreceptor drums (for instance, refer to Japanese Patent Unexamined Publication No. 2005-234229 bulletin).
In conventional image forming apparatus, the photoreceptor drums and the plurality of intermediate transfer rollers have been disposed at such positions that contact is made between each of the photoreceptor drums and each of the intermediate transfer rollers sandwiching the intermediate transfer belt in between, and then each of the intermediate transfer rollers to which a transfer bias is applied causes the intermediate transfer belt to contact with pressure each of the photoreceptor drums. In this case, because each of the photoreceptor drums has a zone of contact in common with the opposed intermediate transfer roller in the direction of movement of the intermediate transfer belt, it is likely that, depending on a nip pressure to each of the photoreceptor drums, missing characters or the like occurs due to toner aggregation, thus resulting in deficiencies in picture quality.
To overcome this problem, for example, as shown in FIG. 6, in the transfer device of the Japanese Patent Unexamined Publication No. 2005-234229 bulletin, photoreceptor drums 31A-31D are disposed in the order of the photoreceptor drum 31D, the photoreceptor drum 31C, the photoreceptor drum 31B and the photoreceptor drum 31A from the upstream side toward the downstream side along a direction of movement C of the intermediate transfer belt. And, in the transfer device, the photoreceptor drums 31A-31D have a zone of contact in common with the opposed intermediate transfer rollers 34A-34D in the direction of movement of the intermediate transfer belt 41 respectively. Then, a transfer bias is applied from each of the intermediate transfer rollers 34A-34D to each of the photoreceptor drums 31A-31D through the intermediate transfer belt 41. In this case, assuming that a direction of rotating shafts of the respective photoreceptor drums 31A-31D is arranged in a horizontal direction, because the bottom of an idle roller 43 is located above the tops of the photoreceptor drums 31A-31D, and since the intermediate transfer belt 41 is stretched obliquely onto the photoreceptor drum 31D of the most upstream side from an upper direction toward a lower direction, a nip pressure to the photoreceptor drum 31D of the most upstream side is not stable. As a result, transfer failure occurs at the photoreceptor drum 31D of the most upstream side.
Then, in the transfer device of the Japanese Patent Unexamined Publication No. 2005-234229 bulletin, occurrence of the transfer failure has been overcome by controlling the nip pressure(s) to the respective photoreceptor drums 31A-31D; even so, controlling the nip pressure or pressures is a complicated process.
Also, as shown in FIG. 7, when a distance in horizontal direction is increased between the compliance roller 43 and the photoreceptor drum 31D of the most upstream side while a distance in vertical direction is fixed between the bottom of the compliance roller 43 and the tops of the photoreceptor drums 31A-31D, an entry angle of the intermediate transfer belt 41 to the photoreceptor drum 31D of the most upstream side decreases (is lowered). In this manner, when the entry angle of the intermediate transfer belt 41 is decreased (lowered), the occurrence of transfer failure of the photoreceptor drum 31D of the most upstream side is overcome. Nevertheless, it follows from this that the device upsizes as much as the distance in horizontal direction is increased between the photoreceptor drum 31D of the most upstream side and the compliance roller 43.
Further, as shown in FIG. 8, when a supplementary roller 44 is provided separately on the upstream side of the photoreceptor drum 31D of the most upstream side, the intermediate transfer belt 41 is depressed by the supplementary roller 44 to the photoreceptor drum 31D side. In this manner, by decreasing (lowering) the entry angle of the intermediate transfer belt 41 to the photoreceptor drum 31D of the most upstream side, the occurrence of transfer failure of the photoreceptor drum 31D of the most upstream side is overcome. However, it follows from this that the supplementary roller 44 and a lifting means to move the supplementary roller 44, etc. become necessary separately, thus resulting in an increased number of parts, and that the device upsizes because an area for housing the lifting means becomes necessary.
Thus, the present invention is directed to providing a transfer device capable of decreasing the occurrence of deficiencies in picture quality while realizing downsizing thereof.